1. Field of the Invention
The invention relates to photoelectrochemical electrodes and, more particularly, to the utilization of photoelectrochemical electrodes for producing electricity from semiconductor liquid junction photovoltaic cells and for producing stored energy products from photoelectrosynthetic and photoelectrocatalytic cells.
2. Description of the Prior Art
Three photoelectrochemical cells that have recently received considerable attention are photovoltaic semiconductor liquid junction cells, photoelectrosynthesis and photoelectrocatalysis. Such cells utilize photoelectrodes to convert energy from light, usually solar energy, into other forms, such as electrical or chemical energy. Typically, solid semiconductor photoelectrodes are employed to facilitate the conversion. These solid materials, according to the band theory of solids, contain atoms whose discrete electronic energy states have merged into energy bands of allowed energies for electrons. The energy required to excite electrons in such solid materials from a maximum energy in the valence band to a minimum energy in the conduction band represents the band gap energy. At approximately room temperature, valence and conduction energy bands of conductors such as metallic solids are not separated, i.e. they have a band gap of about 0. Furthermore, the valence and conduction bands of semiconductor solid materials are typically separated by a band gap of above 0 to less than about 4.0 e.V., while higher values are associated with insulator materials.
The most efficient utilization of terrestrial solar energy by semiconductor materials has been observed to occur with the absorption of photons associated with near-infrared light. Light-absorbing semiconductor materials having a band gap of approximately 1.4 e.V. tend to maximize the efficiencies of the conversion from solar to other forms of energy. In photoelectrosynthesis, photoelectrocatalysis and photovoltaic liquid junction cells, electron-hole pairs are generated by the absorption of light in either semiconductor photoanodes, semiconductor photocathodes or both. The electron and the hole of electron-hole pairs are separated at a semiconductor-liquid junction and are injected at the respective electrodes to produce electrochemical oxidation and reduction reactions. Ordinarily, holes move to the surface at n-type semiconductors and induce oxidation reactions while electrons move to the surface at p-type semiconductors and cause reduction reactions.
One of the problems with photocatalytic electrodes containing semiconductor materials is stability. Photogenerated holes at an n-liquid interface and photogenerated electrons at a p-liquid interface are often capable of respectively oxidizing and reducing the semiconductor material themselves. Such a stability problem appears more acute with n-type materials where the photogenerated holes at the n-liquid interface are capable of oxidizing the semiconductor.
A number of approaches have been taken to stabilize semiconductors. In one approach, by proper choice of a redox-couple, the photogenerated holes may be removed rapidly before decomposition can occur. Other approaches involve changing from an aqueous liquid to a non-aqueous one or modifying the semiconductor surfaces. Another approach includes putting a coating material, such as a highly conductive metal like platinum, or a semiconductor having a wide band gap (i.e. &gt;3.0 e.V.), such as SnO.sub.2, or a polymer such as polypyrrole, on a photoelectrode surface. However, the search continues for coating materials that impart stability to semiconductor-containing photoelectrodes employed in highly efficient photovoltaic and photoelectrochemical cells.
Accordingly, it is an object of the present invention to provide a photoelectrode that is highly stable, especially when utilized in photoelectrochemical cells.
Another object of the invention is to provide a coating material for a semiconductor-containing photoelectrode that improves the efficiency of a photoelectrochemical cell.
Yet another object still is to provide a method for producing a highly stable photoelectrode that may be employed in a photovoltaic, photoelectrosynthetic or photoelectrocatalytic cell.
A further object of the invention is to provide a photoelectrochemical cell employing a novel photocatalytic electrode.
These and other objects and advantages of the invention will become apparent from the following description.